Field of the invention
Reactive power compensation regulation devices are needed in mains supplies subject to high current disturbances, for example in parts of mains supplies near arc furnaces. For this purpose it is necessary to use devices for measuring the power absorbed by the electrical circuit and if action is to be taken immediately to attenuate voltage fluctuations on the mains supply the measuring device must have a very fast response.
The frequency spectrum of the instanteous power of an arc furnace is represented by a continuous curve, the amplitude of which is high in a range of frequencies between 0 and approximately 25 Hz.
It is known that high-amplitude power fluctuations at these low frequencies cause a phenomenon known as "flicker" which is characterized by fluctuations in the brightness of incandescent lamps when their supply voltage is amplitude modulated.
The well-known thyristor controlled reactive power compensators are capable of significantly attenuating these phenomena, provided that there are available measured power or voltage signals giving a faithful representation of the fluctuations to be compensated in the frequency band considered, in this instance approximately 0 to 25 Hz, without significant phase shift in this frequency band or unwanted noise outside this frequency band.
Measuring the reactive power or voltage by known means enables only one of these objectives to be achieved at a time.
A known measuring system comprises a diode discriminator yielding a signal proportional to the absolute value of the voltage or a quadratic demodulator yielding a signal proportional to the square of the voltage or a reactive power demodulator followed by a lowpass filter.
The transfer function is of the lowpass type, the order depending on the required frequency range limit. It is possible to obtain a very rapid roll-off in the curve of the frequency spectrum from the cut-off frequency and the signals at the output of the device are of sufficient quality for comparative measurements but the phase differences between the various frequencies in the range considered at the output and input of the device are too high and as a result the output signal is not suitable for any form of fast-acting automatic control system.
An object of the present invention is to overcome this disadvantage.